Fracture Development at Laminated Floor Layers Under Longwall Face in Deep Coal Mining

Abstract

The increment of stress level and fracture depth in deep longwall mining can cause groundwater inrush accidents frequently, and it is essential to study the characteristics of fracture development around laminated rock layers at the floor. This can help to understand the mechanism of groundwater inrush events and to reduce the potential risk. According to the rock mass stress–strain curve and the stress redistribution around the floor area in a deep longwall face, this study focused on the failure mechanisms in this area. A physical longwall model was established to study fracture development at the unloading zone around the floor area. The results showed that the plastic fracture at the floor was generated by high compressive stress around the floor and coal rib area after the breakage of main roof. The unloading starting point of stress changed nonlinearly, and the unloading stress increased nonlinearly. Therefore, the unloading fracture depth and the floor heave deformation can be much larger than those in shallow mining. In addition, the horizontal bedding plane can accelerate fracture development. When the unloading stresses increase, the branch fractures develop downward and connect the original joints and bedding planes at deeper floors. The dominant angle of main cracks varies in the range of 50°–85°, and the number of branch fractures can reduce when the dip angle of the main cracks increases (close to 85°). In addition, the direction of main fractures was nearly vertical, and it was hard to connect them to the horizontal joints and bedding planes.